The present invention relates to the production of ethers, optionally with the co-production of diols and/or lactones by reaction of an organic feed material in the presence of hydrogen. The reaction will generally be by hydrogenation and/or dehydration. The organic feed material is selected from dicarboxylic acids and/or anhydrides, monoesters of dicarboxylic acids and/or anhydrides, diesters of dicarboxylic acids and/or anhydrides, lactones, a mixture thereof or a mixture of two or more thereof. In particular it relates to the production of C4 to C12 ethers, optionally with the co-production of the corresponding diols and/or lactones by the reaction of di-(C1 to C4)alkyl esters of C4 to C12 dicarboxylic acids and/or anhydrides in the presence of hydrogen. More particularly, it relates to the production of cyclic ethers.
More particularly, the present invention relates to a process for the co-production of C4 compounds, more specifically tetrahydrofuran, butane-1,4-diol and/or γ-butyrolactone from a hydrocarbon feedstock comprising a dialkyl maleate by vapour phase reaction in a hydrogen rich stream. In a particularly preferred arrangement of the present invention, it relates to a process for the production of at least 20% of tetrahydrofuran with co-production of butane-1,4-diol and/or γ-butyrolactone. In the most preferred arrangement it relates to the production of tetrahydrofuran with any residual butane-1,4-diol and/or γ-butyrolactone being recycled and converted to further tetrahydrofuran.
It is known to produce diols by hydrogenation of dialkyl esters of dicarboxylic acids and/or anhydrides, lactones, and mixtures thereof with a minor amount, typically no more than about 10 wt/wt % and preferably no more than 1 wt/wt %, of a monoester of the dicarboxylic acid and/or anhydride. Commercial plants have been built which produce butane-1,4-diol as the primary product with small amounts, typically up to about 10 mole %, of tetrahydrofuran and up to about 15 mole % of γ-butyrolactone by hydrogenation of a dialkyl ester of maleic acid and/or anhydride, such as dimethyl maleate or diethyl maleate, which may contain minor amounts of dialkyl fumarate and/or dialkyl succinate. Dimethyl succinate or diethyl succinate have also been suggested as suitable starting materials for hydrogenation to produce butane-1,4-diol, tetrahydrofuran and γ-butyrolactone. These succinates may be formed by any suitable manner and may be from biotechnology sources.
For further information regarding the operation of these plants reference may be made, for example, to U.S. Pat. Nos. 4,584,419, 4,751,334, WO-A-86/03189, WO-A-88/00937, U.S. Pat. Nos. 4,767,869, 4,945,173, 4,919,765, 5,254,758, 5,310,954 and WO-A-91/01960, the disclosure of each of which is herein incorporated by reference.
Whilst many plant operators aim to maximise the yield of butane-1,4-diol and to minimise the yield of the co-products, tetrahydrofuran and γ-butyrolactone, these co-products are themselves valuable commodity chemicals. The tetrahydrofuran is normally recovered as it is an important monomer for making elastomer fibres and is also an important solvent and therefore is a commercially important chemical. The γ-butyrolactone may be recovered but, as the market for this product is small, it is often recycled to the hydrogenation step for conversion to further butane-1,4-diol and the co-product tetrahydrofuran.
The dialkyl maleates which are used as feedstock in such hydrogenation processes may be produced by any suitable means. The hydrogenation of dialkyl maleates to yield butane-1,4-diol is discussed in detail in U.S. Pat. Nos. 4,584,419, 4,751,334 and WO-A-88/00937, which are incorporated herein by reference.
One conventional process for the production of butane-1,4-diol and co-product tetrahydrofuran with optional production of γ-butyrolactone is illustrated schematically in FIG. 1. In this process, a dialkyl ester, such as dimethyl maleate together with any residual methanol from the esterification reactor, is fed via line 1 to a vaporiser 2 where it is vaporised into a stream of hot cycle gas which is usually pre-heated. Cycle gas will normally contain a high concentration of hydrogen gas but may also include other gases including hydrocarbons, carbon oxides, methane, nitrogen. Further, where the cycle gas includes recycled gases from downstream, condensables including product ether, methanol, water, co-products and by-products may also be present.
The cycle gas is fed to the vaporiser 2 in line 3. The combined vaporous stream is then passed in line 4 to the reactor 5 where it is reacted to form butane-1,4-diol, tetrahydrofuran and/or γ-butyrolactone. The product stream 6 is cooled and the reaction products are condensed at 7 and separated from the cycle gas before being passed in line 8 to a refining zone 9. Recovered cycle gas is compressed and recycled in line 10. Make-up hydrogen will be added to the recovered cycle gas in line 11 with the enriched cycle gas being fed back to vaporiser 2. In the refining zone 9 the various products are separated and the butane-1,4-diol is removed in line 12 and the tetrahydrofuran in line 13. The γ-butyrolactone, together with the intermediate dimethyl succinate and some butane-1,4-diol may be recycled in lines 14 and 15. In one arrangement the γ-butyrolactone may be partially extracted in an optional refining zone 16 and removed in line 17. The methanol water stream separated from the product mix will be recycled upstream via line 18.
A significant portion of the butane-1,4-diol produced by this or other conventional methods is subsequently converted to tetrahydrofuran. This conversion step has substantial cost implications both in investment and operation of the plant required for the conversion and as the importance of tetrahydrofuran increases together with its use in derivative applications, it is desirable to provide a process for the production of tetrahydrofuran without the need for this expensive downstream processing. The downstream processing of conventional methods includes recovering the butane-1,4-diol, reacting it to form the tetrahydrofuran and then refining the tetrahydrofuran product.
In conventional processes, the quantity of cycle gas required to vaporise the feed is determined by a number of parameters including the operating pressure, the desired reaction temperature, the vaporiser exit temperature and the vapour pressure of the components to be vaporised.
Whilst it may be desirable to minimise the amount of cycle gas required, with prior art systems, this decrease will require the exit temperature from the vaporiser to be maintained high. However, maintaining a high vaporisation exit temperature would mean that the reaction temperature would be higher than desired. It is desirable to maintain the operating temperature as low as possible for several reasons including avoidance of hydrogen embrittlement of carbon steel equipment, avoidance of excessive catalyst deactivation and to minimise the formation of by-products such as butanol.
It will therefore be understood that the amount of cycle gas required for the reaction is determined by the vaporiser exit temperature and is therefore a compromise between the high temperature necessary to minimise the cycle gas required to vaporise the feed and the relatively low temperatures required for the reasons given above.
In the particular prior art system of the type illustrated in FIG. 1, in which the butane-1,4-diol is the main product, at a reactor inlet temperature of about 165° C. and a pressure of about 63 bar approximately 240 moles of cycle gas are required per mole of dimethyl maleate to be vaporised. Although the temperature will rise across the reactor, the reactor outlet stream will have about the same degree of saturation as the inlet stream because the vapour pressure of the butane-1,4-diol is less than that of the dimethyl maleate in the feed. Since the byproduct γ-butyrolactone and intermediate dimethyl succinate, together with the associated butane-1,4-diol are conventionally recycled to the reaction system, additional cycle gas is required to vaporise the recycle stream(s). This will typically increase the cycle gas requirements to about 310 mols of cycle gas per mole of the dimethyl maleate vaporised, which it will be understood is a significant increase.
Typically a process of the type illustrated in FIG. 1 will produce up to approximately 10 mole % tetrahydrofuran.
It is therefore desirable to provide a process for the production of higher mole % of tetrahydrofuran without the need for expensive downstream processing. It is further desirable to provide a process in which the cycle gas requirements are minimised such that investment and operating costs are reduced as the selectivity to tetrahydrofuran is increased.